Losses of muscular strength and muscle mass with age (sarcopenia) and gains with strength training are well documented, but these responses vary substantially among individuals. Environmental factors, such as differences in exercise levels, do not explain the large inter-individual differences in strength and muscle mass observed with aging or strength training. This suggests that genetic factors may account for at least a portion of these inter-individual differences. Genes that appear to be good candidates for aging- and strength training-induced changes in muscle mass include growth hormone (GH), insulin-like growth factor-I (IGF-I), IGF-I receptor, IGF binding protein (IGFBP)-2 and 4, myostatin, and components of the dystrophin-glycoprotein complex. However, specific genes have not been identified. Thus, it is hypothesized that the effects of age and strength training on muscular strength and muscle mass are affected by polymorphic variations at these critical skeletal muscle-related gene loci. To test this hypothesis genotypes will be determined cross-sectionally in 800 subjects across the adult life span from the Baltimore Longitudinal Study of Aging (BLSA). All of these subjects will have already been tested for strength and most (~600 subjects) will have been tested for muscle mass by the time this study begins. Genotypes also will be determined in 100 subjects who have completed a standardized strength training program in previous longitudinal studies in this laboratory. In the BLSA study, strength and muscle mass will be analyzed by general linear model procedures to fit the effects of genotype, gender, and age as regression variables and the interactions of these variables. In the strength training study, linear mixed model procedures will be used with the fixed portion containing the effects of genotype, strength training, and their interactions. The potential significance of this study is that it may identify subjects prone to early onset of sarcopenia and provide information about who might be most responsive to strength training interventions designed to reverse the effects of sarcopenia.